1. Field of the Invention
The present invention is directed to angioplasty catheters and specifically to the sheathing of such catheters.
2. Description of the Prior Art
Balloons on dilation catheters are provided with a sheath to protect the balloon prior to use and, more importantly, to "cold flow" the balloon material into a low collapsed profile. Closely fitted pieces of tubing made from low friction material such as tetrafluoroethylene (TFE) have been used for this purpose. These tubes, however, have several disadvantages.
First, the tube must be pushed over the length (2-3 cm.) of the balloon portion of the catheter without damaging the balloon. Although the balloon portion is visually inspected before insertion into the body, any weakening of the balloon wall will be detected only upon inflation of the balloon within the patient.
Second, clearance must be provided in order to avoid damage. The greater the clearance between the collapsed balloon profile and the inside of the tube, the greater the eventual diameter of the collapsed balloon due to progressive "cold flow" of the balloon material in storage. In addition to the sliding clearance needed, the tube has a manufacturing tolerance of, for example, at least plus or minus 0.001 inch. In a low profile balloon having a minimum collapsed diameter of 0.030 inch where a minimum sliding clearance of 0.001 inch is required, the tube with a manufacturing tolerance will cause the desired profile to vary by as much as 0.003 inch, i.e., ten percent of the minimum collapsed diameter of the balloon. If one further allows a tolerance for catheter components that can vary, the variance from the desired profile can be even greater.
During the time in which the balloon portion resides within the outer tubing or sheath, the balloon portion will take a set, i.e., its material will "cold flow", and the outside diameter of the collapsed balloon portion will approximate the inside diameter of the outer tube. The smaller the outer diameter of the balloon portion of the catheter, the smaller the critical artery location into which it may be inserted. It would be advantageous for angioplasty procedures to have a more compact collapsed profile than that which conventionally results from the tubing arrangement discussed above.
U.S Pat. Nos. 4,710,181 and 4,738,666 disclose variable diameter catheters which are folded in order to reduce the diameter for convenience and for less traumatic insertion into a body orifice. In these devices, the catheter is maintained in a compressed condition by means of the external sheath which is removed after the catheter is placed in the body orifice. In angioplasty devices, the use of an outer sheath within the body requires an even greater opening size to accommodate the dimensions of the sheath. It would be beneficial to have some method of compacting a collapsed angioplasty balloon other than by compressing it and inserting it into a sheath with a fixed dimension which then must also be inserted into the body.
U.S. Pat. No. 4,563,176 discloses the use of a flexible protective sheath in the form of a plastic bag which encloses a catheter as a sterile storage device. The sheath is removed before insertion of the catheter, but, unfortunately, does not compact the catheter to a minimum insertion profile. It would be beneficial to have a method of compacting a collapsed angioplasty balloon portion of a catheter both for eventual insertion and for maintaining the device for storage purposes.